The present invention relates in general to the field of proton exchange membrane (xe2x80x9cPEMxe2x80x9d) fuel cell stack assemblies, and more particularly, to an improved polarized gas separator for use in the bipolar construction of fuel cell stack assemblies.
A fuel cell is an electrochemical device that converts fuel and oxidant into electricity and a reaction by-product through an electrolytic reaction that strips hydrogen molecules of their electrons and protons. Ultimately, the stripped electrons are collected into some form of usable electric current, by resistance or by some other suitable means. The protons react with oxygen to form water as a reaction by-product.
Natural gas is the primary fuel used as the source of hydrogen for a fuel cell. If natural gas is used, however, it must be reformed prior to entering the fuel cell. Pure hydrogen may also be used, if stored correctly. The products of the electrochemical exchange in the fuel cell are DC electricity, liquid water, and heat. The overall PEM fuel cell reaction produces electrical energy equal to the sum of the -separate half-cell reactions occurring in the fuel cell, less its internal and parasitic losses. Parasitic losses are those losses of energy that are attributable to any energy required to facilitate the ternary reactions in the fuel cell.
Although fuel cells have been used in a few applications, engineering solutions to successfully adapt fuel cell technology for use in electric utility systems have been elusive. The challenge is for the generation of power in the range of 1 to 100 kW that is affordable, reliable, and requires little maintenance. Fuel cells would be desirable in this application because they convert fuel directly to electricity at much higher efficiencies than internal combustion engines, thereby extracting more power from the same amount of fuel. This need has not been satisfied, however, because of the prohibitive expense associated with such fuel cell systems. For example, the initial selling price of the 200 kW PEM fuel cell was about $3500/kW to about $4500/kW. For a fuel cell to be useful in utility applications, the life of the fuel cell stack must be a minimum of five years and operations must be reliable and maintenance-free. Heretofore known fuel cell assemblies have not shown sufficient reliability and have disadvantageous maintenance issues. Despite the expense, reliability, and maintenance problems associated with heretofore known fuel cell systems, because of their environmental friendliness and operating efficiency, there remains a clear and present need for economical and efficient fuel cell technology for use in residential and light commercial applications.
Fuel cells are usually classified according to the type of electrolyte used in the cell. There are four primary classes of fuel cells: (1) proton exchange membrane (xe2x80x9cPEMxe2x80x9d) fuel cells, (2) phosphoric acid fuel cells, and (3) molten carbonate fuel cells. Another more recently developed type of fuel cell is a solid oxide fuel cell. PEM fuel cells, such as those in the present invention, are low-temperature, low-pressure systems, and are, therefore, wellsuited for residential and light commercial applications. PEM fuel cells are also advantageous in these applications because there is no corrosive liquid in the fuel cell and, consequently, there are minimal corrosion problems.
Characteristically, a single PEM fuel cell consists of three major components-an anode gas dispersion field (xe2x80x9canodexe2x80x9d); a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d); and a cathode gas and liquid dispersion field (xe2x80x9ccathodexe2x80x9d). As shown in FIG. 1, the anode typically comprises an anode gas dispersion layer 106 and an anode gas flow field 104; the cathode typically comprises a cathode gas and liquid dispersion layer 110 and a cathode gas and liquid flow field 112. In a single cell, the anode and the cathode are electrically coupled to provide a path for conducting electrons between the electrodes through an external load. MEA 108 facilitates the flow of electrons and protons produced in the anode, and substantially isolates the fuel stream on the anode side of the membrane from the oxidant stream on the cathode side of the membrane. The ultimate purpose of these base components, namely, the anode, the cathode, and MEA 108, is to maintain proper ternary phase distribution in the fuel cell. Ternary phase distribution as used herein refers to the three simultaneous reactants in the fuel cell, namely, hydrogen gas, water vapor, and air. Heretofore known PEM fuel cells, however, have not been able to efficiently maintain proper ternary phase distribution. Catalytic active layers 100 and 102 are located between the anode, the cathode, and the electrolyte. The catalytic active layers 100 and 102 induce the desired electrochemical reactions in the fuel cell. Specifically, catalytic active layer 100, the anode catalytic active layer, rejects the electrons produced in the anode in the form of electric current. The oxidant from the air that moves through the cathode is reduced at the catalytic active layer 102, referred to as the cathode catalytic active layer, so that it can oxidate the protons flowing from anode catalytic active layer 100 to form water as the reaction by-product. The protons produced by the anode are transported by the anode catalytic active layer 100 to the cathode through the electrolyte polymeric membrane.
The anode gas flow field and cathode gas and liquid flow field are typically comprised of pressed, polished carbon sheets machined with serpentine grooves or channels to provide a means of access for the fuel and oxidant streams to the anode and cathode catalytic active layers. The costs of manufacturing these plates and the associated materials costs are very expensive and have placed constraints on the use of fuel cells in residential and light commercial applications. Further, the use of these planar serpentine arrangements to facilitate the flow of the fuel and oxidant through the anode and cathode has presented additional operational drawbacks in that they unduly limit mass transport through the electrodes, and therefore, limit the maximum power achievable by the fuel cell.
One of the most problematic drawbacks of the planar serpentine arrangement in the anode and cathode relates to efficiency. In conventional electrodes, the reactants move through the serpentine pattern of the electrodes and are activated at the respective catalytic layers located at the interface of the electrode and the electrolyte. The actual chemical reaction that occurs at the anode catalytic active layer is: H2 ⇄2H30 +2exe2x88x92. The chemical reaction at the cathode catalytic active layer is: 2H++2exe2x88x92+xc2xdO2⇄H2O. The overall reaction is: H2+xc2xdO2 H2O. The anode disburses the anode gas onto the surface of the catalytic active layer, which is comprised of a platinum catalyst electrolyte, and the cathode disburses the cathode gas onto the surface of the cathode catalytic active layer of the electrolyte. However, when utilizing a conventional serpentine construction, the anode gas and the cathode gas are not uniformly disbursed onto these layers. Nonuniform distribution of the anode and cathode gas at the membrane surfaces results in an imbalance in the water content of the electrolyte. This results in a significant decrease in efficiency in the fuel cell.
The second most problematic drawback associated with serpentine arrangements in the electrodes relates to the ternary reactions that take place in the fuel cell itself. Serpentine arrangements provide no pressure differential within the electrodes. This prohibits the necessary ternary reactions from taking place simultaneously. This is particularly problematic in the cathode as both a liquid and a gas are transported simultaneously through the electrode""ss serpentine pattern.
Another shortcoming of the conventional serpentine arrangement in the anode in particular is that the hydrogen molecules resist the inevitable flow changes in the serpentine channels, causing a build-up of molecular density in the turns in the serpentine pattern, resulting in temperature increases at the reversal points. These hot spots in the serpentine arrangement unduly and prematurely degrade the anode catalytic active layer and supporting membrane.
In the typical PEM fuel cell assembly, a PEM fuel cell is housed within a frame that supplies the necessary fuel and oxidant to the flow fields of the fuel cell. These conventional frames typically comprise manifolds and channels that facilitate the flow of the reactants. However, usually the channels are not an integral part of the manifolds, which results in a pressure differential along the successive channels. FIG. 2 is an illustration of a conventional frame for the communication of the reactants to a fuel cell. This pressure differential causes the reactants, especially the fuel, to be fed into the flow fields unevenly, which results in distortions in the flow fields, ultimately causing hot spots. This also results in nonuniform disbursement of the reactants onto the catalytic active layers. Ultimately, this conventional method of supplying the necessary fuel and oxidant to a fuel cell results in a very inefficient process.
As a single Proton Exchange Membrane (xe2x80x9cPEMxe2x80x9d) fuel cell produces only about 0.30 to 0.90 volts D.C. under a load, the key to developing useful PEM fuel cell technology is being able to scale-up current density in individual PEM fuel cell assemblies to produce sufficient current for larger applications without sacrificing fuel cell efficiency. To accomplish this, practical PEM fuel cell plants have been built from multiple fuel cell assemblies stacked together. Practical stacks generally consist of 20 or more cells in order to produce the direct current voltages necessary for efficient inverting to alternating current. To form stack assemblies, individual fuel cell assemblies are electrically connected to form xe2x80x9cnodes,xe2x80x9d which are then electrically connected to form xe2x80x9cfuel cell stacksxe2x80x9d or xe2x80x9cfuel cell stack assemblies.xe2x80x9d Nodes may be connected in series or parallel to efficiently increase overall power output. When connecting these nodes, to reduce the number of parts and to minimize costs, bipolar separator plates are used between adjacent individual cells. More specifically, in these bipolar plates, one side of the plate serves as the anode side and services the anode side of one fuel cell assembly; and the other side of the plate serves as the cathode side and services the cathode side of another fuel cell assembly. This is often described as bipolar construction. A barrier to free water and gas flow between the two sides of the bipolar plate exists to keep the hydrogen gas flow and the oxygen gas flow separate from one another. A bipolar plate also collects electrons liberated at one electrode, conducts the electrons through the barrier plate, and delivers the electrons to the face of the other electrode on the opposing side of the barrier.
Conventional bipolar plates used in the bipolar construction of fuel cell stacks generally contain three elements: a first gas flow field, an internal barrier or separator plate, and a second gas flow field, specifically a gas and liquid flow field (both referred to in the following generically as flow fields). In conventional assemblies, the flow fields are made from sheets constructed from metals, such as titanium, nickel, and stainless steel, or nonmetallic conductors, such as graphitic carbon. These sheets typically have a series of channels or grooves machined into their surfaces that allow for the passage of gases and liquids such as those described above. These channels or grooves are usually arranged in a serpentine-like pattern, and thus, are subject to the difficulties and problems described above.
The internal barriers or separator plates of conventional assemblies are often fabricated from graphite or metal, such as stainless steel, titanium, or nickel as well. In conventional designs, the internal barrier or separator plate and the gas flow fields are mechanically bonded together. The bonding mechanism has historically been sintering.
Whenever the gas flow fields are bonded to the internal barrier or separator plate by mechanical means, the electrical resistance in the bipolar plate increases, which leads to efficiency losses in the fuel cell stack assembly. Typically, the resistance in such bipolar plates is at best 3.4 ohms. This is undesirable, as excessive resistance diminishes the power potential of the fuel cell stack. The mechanical bonding may also be problematic if not completely accurate because air gaps may be created between the flow fields and the internal barrier. Another problem with conventional bipolar plates is that the mechanical bonding process can introduce contaminants, which are primarily oxidants, into these metallurgical bonds between the flow fields and the internal barrier, which thereby contaminate the system and cause efficiency losses. This is also undesirable.
Additionally, conventional PEM fuel stacks often have poor water management in the cathode, which leads to flooded conditions in the cathode flow field. Flooding occurs when the free water (that is produced as a by-product of the electrochemical reaction) is not removed efficiently from the system. Flooding is particularly problematic because it impairs the ability of the oxygen reactant to adequately diffuse to the cathode catalytic active layer. This significantly increases the internal resistance of the cathode, which ultimately limits the cell voltage potential.
Heretofore known bipolar plates have also failed to manage the liquid-phase water that develops in the anode. As a by product of humidification conventional bipolar plate manufacturers have never recognized the presence of a liquid water management problem in the anode, conventional wisdom holds that water is in vapor form only on the anode side. In the development of the present invention, however, it has been discovered that in low pressure and low temperature PEM fuel cell systems, liquid-phase water exists in its liquid form on the anode side in addition to the free water produced as a by-product of the electrochemical exchange on the cathode side. This liquid-phase water in the anode creates problems in diffusion on the anode side that have yet to be recognized or addressed in conventional systems. These problems are detrimental to performance. To optimize fuel cell stack performance, liquid water management must be addressed on the anode side as well as on the cathode side. Conventional systems have not done this, and this lack of proper water management has led to an inherent loss in fuel cell stack assembly performance potential.
Conventional bipolar plate assemblies have additional problems in that they are difficult to manufacture. Further, strict quality control is required to ensure that the barrier between the two flow fields adequately seals one reactant from the other. These two factors make mass production an impossibility for conventional bipolar assemblies. Also, since conventional bipolar plate designs include three individual components, these conventional designs have problems related to internal electrical continuity and connectivity among the three parts. Adequate continuity and connectivity are difficult to achieve because the mechanical connections among the three individual components not only introduce additional electrical resistance to the system, but also require extreme precision in manufacturing.
Accordingly, there is a need for a bipolar flow field assembly that is suitable for use in bipolar construction of a fuel cell stack assembly, that introduces minimal resistance to the system, that separates the reactant gases completely, that effectively manages heat and produced water on both sides of the bipolar assembly itself, and that can be mass produced efficiently and economically.
Accordingly, the present invention provides a polarized gas separator comprising a flow field that may function as a bipolar flow field mechanism useful in the bipolar construction of a fuel stack assembly that is not only easy to manufacture in a mass production operation, but also reduces resistance, increases cell power potential, effectively manages heat and water within the separator itself, and facilitates a more effective distribution of the reactants to their respective catalytic active layers. It is greatly improved over conventional bipolar plate assemblies commonly used in bipolar construction, as it is a single component system having improved electrical connectivity and conductivity.
Most broadly, the present invention comprises a polarized gas separator comprising a porous conductive substrate and a barrier layer, the barrier layer having a first side and a second side, and laterally extending through the porous conductive substrate so that a first portion of the porous conductive substrate extends from the first side of the barrier layer and a second portion of the porous conductive substrate extends from the second side of the barrier layer. Thus, instead of having three individual components mechanically connected, the present invention provides a single component laterally bisected to effectively create a three-component system without disturbing the electrical properties of the single component. The polarized gas separator of the present invention is greatly improved over conventional bipolar plate assemblies as it is a single-component system rather than a composition of three components. Thus, instead of having two flow fields independently connected to an internal barrier component, the present invention provides one layer of material in a configuration that allows it to function as both an anode flow field and a cathode flow field, but the component itself is unbroken. Because the polarized gas separator of the present invention is a single component, it avoids the resistance problems encountered in manufacturing conventional assemblies. In fact, it is now possible to achieve 1.5xc3x971031 4 ohms of resistance or less within the separator. This is a great improvement over conventional bipolar plate assemblies, which have an internal resistance of at least 3.4 ohms. This single component system also increases connectivity and conductivity within the separator itself. Furthermore, the separator of the present invention is able to effectively seal the reactant gases from each other. Because it is a single component, its manufacture is suitable for mass production operations that are both efficient and effective. Residential applications for fuel cell stack assemblies utilizing the polarized gas separator for the present invention are, therefore, a reality. Specifically, the polarized gas separator of the present invention may be used in conjunction with the distribution frame and fuel cell stack assemblies described in Application Ser. No. 09/669,344, filed Sep. 26, 2000, now U.S. Pat. No. 6,531,238.
The different embodiments of the polarized gas separator of the present invention illustrate that a variety of materials may be used in a variety of selected configurations for the flow fields to achieve the beneficial effects of the present invention, including materials that are commonplace, and therefore, much less expensive than construction materials for conventional bipolar plate assemblies. Suitable materials for the porous conductive substrate include, but are not limited to: metal wool; wire mesh; three-dimensional open-cell foamed structures; carbon filaments; other conductive metal substrates; and aerated nickel wool. Suitable construction materials for the materials include: hastelloys, inconel, nickel, conductive plastics, metal composites, metal and plastic composites, plastic composites, tin oxides, stainless steel, and their derivatives. Essentially, any material that has sufficient porosity and surface texture is suitable in the present invention. A typical construction material for the barrier layer is a thermal-setting epoxy. The flexibility of the design of this polarized gas separator enhances the desirability of the present invention, especially as the push for affordable fuel cell operations for residential use increases.